1. Field of the Invention
This invention relates to methods and apparatus for electronically diagnosing and analyzing the performance of internal combustion engines. The invention relates particularly to digital engine analyzers of the type which display digitized information on an oscilloscope screen.
2. Description of the Prior Art
The present invention is an improvement of the digital engine analyzer disclosed in U.S. Pat. No. 4,800,378. One of the waveforms commonly analyzed in engine analyzers of that type is the secondary ignition pattern, which has a distinctive shape. The pattern includes a high-amplitude spike of very short rise and fall times at the beginning of the cylinder power stroke, caused by the buildup of voltage across the spark plug just prior to its firing; a plateau region of medium amplitude, which is the "burn time" when the spark plug is actually firing; and an oscillatory or "ringing" portion after termination of the spark plug firing. It will be appreciated that the power stroke is the movement of the piston away from the spark plug in response to combustion of the fuel, thereby delivering power to the crankshaft.
Heretofore the spark plug burn time has rarely been used as a diagnostic aid. However, useful diagnostic information can be gained by determining the spark plug burn time, and particularly by comparing the spark plug burn times of the several cylinders. For example, short burn times may be indicative of fouled plugs and/or high resistance in the secondary of the ignition. Comparisons of burn times can determine if one or more cylinders may have faulty spark plugs and/or high resistance in the ignition components.
It is known to measure the spark plug burn time, one system for doing so being disclosed in U.S. Pat. No. 4,291,383. But such prior art systems simply display numerical data for the spark plug burn times, which makes it difficult to readily compare the burn times of the several cylinders. Furthermore, such systems require the use of input signals from both the primary and the secondary of the ignition coil in order to derive the burn time information.
One of the principal diagnostic techniques utilized in prior engine analyzers is the display in kilovolts of the peak voltage across the spark plug for each cylinder firing. In the aforementioned U.S. Pat. No. 4,300,378, this information is displayed in a number of ways, viz., a display of the secondary waveform itself, a bar graph of the peak values for the several cylinders, and the display of numerical minimum and maximum values for each cylinder. But none of these techniques permits the analysis of the peak voltage performance of a single cylinder over time, independently of the other cylinders.
Another diagnostic technique used in prior engine analyzers is cylinder shorting, i.e., shorting out the ignition voltage to a selected cylinder. The purpose of cylinder shorting is to determine the contribution of each individual cylinder to the overall power output of the engine by successively shorting, or preventing operation of, selected cylinders and noting the effect on the speed of the engine. If the cylinder were contributing no power, then the shorting of that cylinder would not decrease the engine speed. If, on the other hand, the individual cylinder being shorted were a normal contributor to the overall power, then the speed of the engine would drop in response to the shorting. If each individual cylinder contributed the same amount to the overall power, then the shorting of each cylinder would result in substantially the same speed drop.
In modern computer-controlled engines with catalytic converters it is difficult and potentially harmful to short out cylinders. Indeed, engine manufacturers specifically warn against the use of this technique. Accordingly, at least one prior system has obtained an indication of the cylinder-by-cylinder power contribution or power "balance" without shorting the cylinders, by means of measuring the variations in firing times between the cylinders. The system provides a bar graph of the variations in firing times between the cylinders. But this display shows the time variation for each firing and, therefore, can experience considerable flutter over several engine cycles, making it difficult to read.
In a digital engine analyzer, the analog engine signals are converted into digital information by sampling the analog waveform at a predetermined rate and generating digital representations of the sample values. That digital information is stored and then displayed on the oscilloscope screen. The oscilloscope is a cathode-ray tube, and the display thereon consists of a multiplicity of dots arranged in horizontal rows with a predetermined number of dots in each row, this number representing the maximum number of samples which can be simultaneously displayed across the screen, which places an upper limit on the resolution of the waveform display. This is not a problem if the amplitude of the analog waveform is relatively constant over time or the rate of change thereof is not great. However, during those portions of the waveform containing very rapid rise and fall times, such as during the spike portion at the beginning of the cylinder ignition waveform pattern, it is difficult or impossible to faithfully represent the actual waveform on a digital oscilloscope wherein an entire cylinder period is to be displayed.
This problem can be solved by increasing the sampling rate, but if all the samples are displayed this would prevent an entire cylinder period from being displayed on the screen. In the aforementioned U.S. Pat. No. 4,800,378, the spike portion of the cylinder ignition waveform is displayed by capturing the analog peak value and digitizing it and then later inserting that value in the displayed waveform. But the insertion doesn't occur until the following engine cycle at the earliest, and a new peak is captured only once every several engine cycles, depending on engine speed, so that the displayed waveform is not a true "live" waveform. Furthermore, the peak value might get inserted at the wrong point, resulting in distortion of the waveform.
Most modern automobile engines have on-board computers which control and/or monitor a number of different engine parameters and which produce a serial data stream indicating the status of monitored parameters. This data stream may be accessed through the Assembly Line Data Link (ALDL) connector on the engine. Hand-held diagnostic instruments, known as scanners, are adapted to plug into the ALDL connector and access the serial data stream and interpret and display the information. But such scanners have very limited displays.
U.S. Pat. No. 4,602,127 discloses the concept of interfacing such a scanner with an engine analyzer so that all of the parameter data available to the scanner can be simultaneously displayed on the cathode-ray tube of the analuzer, but the patent does not disclose any means for accomplishing this result. Furthermore, the scanner must be utilized close to the engine, while the engine analyzer may be remotely located and, therefore, it may be difficult for the operator to read the engine analyzer display while at the same time operating the scanner controls.